1. The Field of the Invention
The invention generally relates to Distributed Bragg Reflectors (DBRs). More specifically, the invention relates to methods of reducing DX centers in DBRs to allow for higher n-type doping.
2. Description of the Related Art
Lasers have become useful in a number of applications. For example, lasers may be used in optical communications. Laser light pulses may be used to transmit digital data across a fiber-optic network. A laser may be modulated by an electronic digital signal to produce an optical digital signal that is transmitted on a fiber-optic cable. At a receiver end, a photodiode or other optically sensitive device converts the optical signal to an electronic digital signal. Optical networks allow modern computing devices to communicate at high speeds and over long distances.
One particular type of laser that is used in optical communications is the Vertical Cavity Surface Emitting Laser (VCSEL). As its name suggests, a VCSEL emits light from a surface, typically the top surface, of the laser. The VCSEL has a cavity bounded by a top mirror and a bottom mirror that reflect photons in the cavity. The top and bottom mirrors are generally distributed Bragg reflector (DBR) mirrors. DBR mirrors are formed by alternating a number of layers of materials with higher and lower indices of refraction.
In one example, a VCSEL designed to emit light with a wavelength of 650 to 670 nm is formed on a Gallium Arsenide (GaAs) substrate. The VCSEL may have mirrors that are formed from alternating layers of AlAs and Al0.5Ga0.5As (i.e. 50% Al to 50% Ga ratio). Between the top and bottom mirror is the active region which includes a pn junction to form a diode.
The DBR mirrors may be doped with n and p-type impurities which allows for current to be conducted through the mirrors to provide a bias current for the pn junction in the active region. To reduce the series resistance of the n-type mirror for example, the DBR mirror is heavily doped such that the free carrier concentration is n=1×1018 cm−3.
One phenomenon that exists in n-type mirrors that have an Al concentration of about 40%-60% is known as a DX center. A DX center occurs when an atom in a crystal structure exists where it ordinarily should not exist. This results in a band that exists between the valance band and the conduction band. This intermediate band can absorb free carriers, such as holes and electrons. The free carriers tend to become trapped in the DX center such that they are not available for conducting current or generating light. DX centers cause higher series resistance in VCSELs, higher leakage currents, and a reduction in doping effectiveness. This results in a need to increase bias currents, that supply current to the VCSEL, to obtain a particular optical output level. Ultimately, DX centers reduce the amount of power and power swing available for VCSEL laser applications. This can result in a reduction in the distance that data can be transmitted and an increase in the number of bit errors existing in a digital optical network.